Ms Ardern found out she was expecting a week before Mr Peters announced his party would side with Labour to form the next Government.

It doesn't appear that Mr Peters had any idea she was pregnant - some people on social media suggested at the weekend he only picked Labour because he knew Ms Ardern was pregnant, and he'd get to act as Prime Minister during her absence.

Clarke Gayford & Jacinda Ardern.

Ms Ardern's partner, TV host Clarke Gayford, said during the negotiations and swearing in of the new Government, it was their "little secret".

Ms Ardern said they only told one other person - "a friend of ours, and that was basically it for quite a long time".

Breaking the news to the Deputy Prime Minister was easy.

"I popped around to his home and sat down with him, we had a couple of sandwiches together and talked about it. He was wonderful, wished us congratulations, and then went straight into talking about how we would make it work. It didn't seem to faze him at all."

Ms Ardern said she'll keep a close watch on Mr Peters while she's away.

"It's essentially exactly the same as if I was in another country. He has the powers and functions, but will work in consultation with me on significant issues. I'm going to be around. I'm going to be keeping a close eye on everything that's going on."

In the meantime, Mr Gayford said he's getting ready to be a house-husband by slowly weaning himself off fishing, and taking up knitting.